This application addresses the great need for an animal model that reflects the pathogenic potential of HIV-1. Building upon our experience with the SIVmac and ruminant lentiviruses, we passaged SHIV through macaques and in pigtail macaques, and derived a virulent SHIV swarm that caused persistent, productive infection with a severe precipitous loss of CD4+ T cells. In this proposal, we will examine the pathogenesis of infection in pigtail macaques with this SHIV warm, addressing the hypothesis that loss of CD4+ T cells is caused by viral lysis of the cells, phenomenon evident by apoptotic changes in the lymphoid tissues. We hypothesize two periods of lysis of the T cells which are matched by a decline in CD4+ T cell count in peripheral blood. The first period of decline occurs early after infection and the second after the immune responses to the virus break down. We will determine whether particular sequences in the HIV-1 env gene are associated with the pathogenic effects of the virus and whether such genotypes are selected in particular tissues. Cloning and sequencing of env genes obtained by PCR from tissues, and examination of biological properties of viruses isolated from tissues, and examination of chimeric viruses constructed with the env genes from tissues will be used to answer these questions. We will derive pathogenic molecular clones of SHIV directly from affected tissues. This will provide information on the molecular basis of SHIV pathogenicity and supersede use of limited tissue material for reproducing disease. Finally, we will infect pigtail macaques with SHIV-delta-nef as a vaccine and challenge them with virulent SHIV. Prevention of disease, rather than infection by the virulent virus, is the goal of this experiment. We expect that pigtail macaques infected with the virulent SHIV swarm will develop AIDS, with loss of CD4+ T cells (less than 200/microliter) and opportunistic infections; that apoptosis will be demonstrable in lymphoid issues during the periods of CD4+ loss, and that some macaques will also develop disease in non-lymphoid issues, enabling us to link specific genotypes with particular tissues. We also expect that macaque infected with the SHIV-delta-nef will develop persistent non-pathogenic infection and will not develop disease hen challenged with the virulent SHIV virus swarm.